Fructans are a type of water-soluble carbohydrate whose primary function is to provide a readily accessible energy reserve for plant growth. Fructans are associated with various advantageous characters in grasses, such as cold and drought tolerance, increased tiller survival, enhanced persistence, good regrowth after cutting or grazing, improved recovery from stress, early spring growth and increased nutritional quality.
Fructan synthesis and metabolism in grasses and cereals is complex. Fructans consist of linear or branched fructose chains attached to sucrose. The chain length of plant fructans ranges from three up to a few hundred fructose units. Different types of fructans can be distinguished based on the linkage types present. In perennial ryegrass three types of fructans have been identified: inulins, inulin neoseries and levan neoseries, with four fructosyltransferse (FT) enzymes involved in this fructan profile (FIG. 6). The enzyme 1-SST (sucrose: sucrose 1-fructosyltransferase) catalyses the first step in fructan biosynthesis while the remaining enzymes elongate the growing fructose chain (1-FFT: fructan: fructan 1-fructosyltransferase, 6G-FFT: 6-glucose fructosyltransferase, and 6-SFT: sucrose: fructose 6-fructosyltransferase). The enzymes 1-FEH or 6-FEH (fructoexohydrolase) reduce fructan chain length by releasing fructose molecules.
Fructans represent the major non-structural carbohydrate in 15% of plant species and play a key role in forage quality. Ruminant livestock grazing on high fructan diets show improved animal performance.
In grasses the level and composition of fructans has been increased in stems and leaf sheaths through the engineered expression of fructosyltransferase (FT) genes.
However, manipulating biochemical pathways by manipulating the activity of enzymes in the pathways may be difficult because of the ways in which the various enzymes and their substrates may interact.
Thus, it would be desirable to have improved methods of manipulating biochemical pathways, particularly in plants. For example, it would be desirable to have methods of manipulating fructan biosynthesis in plants, including grass species such as Lolium and Festuca and cereals such as wheat and maize, thereby facilitating the production of eg. forage grasses with improved herbage quality, leading to improved pasture production, improved animal production and reduced environmental pollution, bioenergy grasses with enhanced biomass yield e.g. for bioethanol production, and eg. cereals with increased grain and biomass yield.
Nucleic acid sequences encoding some of the enzymes involved in the fructan biosynthetic pathway have been isolated for certain species of plants. For example, PCT/AU01/00705 to the present applicants, describes fructosyltransferase homologues from Lolium and Festuca. However, there remains a need for materials useful in the modification of fructan biosynthesis in plants, and also to engineer fructan accumulation in different parts of the plant.
It is an object of the present invention to overcome, or at least alleviate, one or more of the difficulties or deficiencies associated with the prior art.